One man's steps to a greener lifestyle.

Monthly Archives: May 2020

For some time now my Home Energy Management System (HEMS) has been managing many of my domestic electricity consumers including:

car charging

dishwasher

home storage battery

washing machine

water heating via immersion heater

Domestic solar panels

The overarching strategy has been to:

maximise use of my own solar energy (rather than consume from the grid)

prioritise consumers for best value within the constraint of available solar generation

when power is needed from the grid to optimise the purchase price by shifting consumption to the cheapest periods (my price changes every half an hour)

For some consumers such as car charging and water heating this has resulted in those consumers switching between two modes:

self-consumption when they are enabled to use the ‘free’ electricity from my own solar panels (subject to device prioritisation) with some proportional control

boost when they run at full power drawing some if not all of the required power from the grid at the lowest available price

However the quite exceptional stress being put on the grid in the UK has prompted some expansion of capability. It’s normal once in a while that my electricity prices go negative, commonly caused by the overlap of large amounts of renewable electricity on the grid (e.g. excess solar output on sunny afternoons and/or high windfarm output due to wind conditions) and low demand (summer nights without heating demand, summer afternoons, bank holidays etc) which is exacerbated by the current corona situation. The current corona situation has made this more common and the plunge pricing events more extreme with multiple hours of negative pricing today some of which are into double digits (which I think is unprecedented). For car charging and water heating this has resulted in a new control mode.

Stepped Mode 3 EVSE in consumer unit case

Hot water cylinder with immersion heater

The new control mode is a disabled status where the the device neither self-consumes nor is forced on. In the short term this increases export to the grid, but the mode is intended to help balance the grid disabling consumption for now to enable more consumption when the grid is under most extreme stress from an excess of generation over demand. Or to think of it another way, it passes up the opportunity to use free electricity now in order to be paid to use electricity later, responding to the price incentive to support balancing the grid.

Thus on a normal day these devices switch half-hourly between self-consumption (free electricity) and Boost (paid for electricity), but on a price plunge day then they switch half hourly between the new disabled state (no electricity, increased export) and the existing Boost (but now paid-to-consume electricity).

The full availability of modes is thus:

mode / other

abbreviation

battery storage

car charger

water heating

Make and model

Powervault G200

N/A

Immersun v2

Control means

API

Relay

Mixed API and relay

Mode – Boost

++

Powervault “Force charge”

via HEMS relay (Ch 1)

Via HEMs relay (Ch 2) to Immersun “External Boost” input.

Mode – Self-consumption only

+

Priority #1Powervault “Charge only”

Priority #2via Immersun relay output (Ch 3)

Priority #3Immersun default behaviour

Mode -Disabled

0

N/A – Available in API but not used by HEMS.

via HEMS relay (Ch 4)

Immersun “Holiday” mode via API

Mode – Both self-consumption and self-discharge available

+/-

Powervault “Normal” via API

N/A – No reverse flow from car to home available (not V2X capable)

N/A

Major device modes available to HEMS

We should thus be better equipped to support the grid in the current circumstances.

Import / Export at the smart meter for Saturday 23rd May

The chart above shows the resulting behaviour. In particular the large negative currents through the morning to early afternoon show that much of the normal self-consumption has been disabled. Then from mid-afternoon the import shows the effect of enabling multiple consumers simultaneously. Here the behaviour of the car charging and water heating was boosted as this point, while the dishwasher’s and washing machine’s existing behaviour added to load.

Not that it has anything to do with the revised controls, but the spikiness of the export during the day shows the highly variable nature of the export through the day being a function of both variable generation through passing clouds and variable consumption with kettle boils and the like. Thus it’s important that consumers for self-consumption have automated closed loop control since manual control of an immersion heater or car charger to achieve high self-consumption with minimal import would have been almost impossible even with a level of human intervention wholly at odds with the scale of savings achieved – small savings hour-by-hour add up over the day, weeks and months but their value is relatively tiny compared to the labour to attempt equivalent control manually.

Here is a similar import-only half-hourly view from the smart meter WAN side:

Half-hourly consumption and cost from smart meter WAN side.

The screenshot above clearly shows those periods of import being targeted at the periods with the most negative prices. Since different consumers need power for different periods of time (for example it takes about 7 hours to charge the battery, but only 2 or 3 for a tank of hot water) then consumption rises as cost falls. My consumption-weighted average cost was -6.22 p/kWh yesterday. However the same price point during the day or night has delivered different consumption from the grid as the output of the solar panels must be consumed before consumption from the grid starts. We are still some way from the point where it becomes economically attractive to disable the solar panels.

Daily electricity cost for 28 days (vertical scale is fractions of a pound)

Finally the above image shows the last 28 days of electricity average cost in p/kWh. Although some other days included some periods of negative pricing, the quite exceptional pricing yesterday is amply illustrated with the combination of extreme prices and the new load management mode delivering revenue (i.e. negative cost) of 82.4 pence through consumption of 13.252 kWh at an average 6.22 p/kWh.

This is something of a zero sum game in terms of consumption as I don’t artificially increase consumption to improve income – such as leaving the oven on with the door open during a summer day – this is all about shifting consumption that would have happened anyway. However we have not only supported the grid when it is most stressed but also reduced our energy costs significantly (to the point of being significantly negative) by moving consumption from being predominately self-consumption (i.e. from our own solar panels) to being predominantly grid consumption.

For some time now I’ve been thinking about creating a real time display which pulls together data from a variety of sources around the home to provide an overview of what’s going on without the need to visit multiple web pages or apps. Until the last 10 days or so that involved little more than thoughts of how I might evolve the existing immersun web page with more content (I don’t have the skills to write my own app), but then about 10 days ago I saw an online gauge that someone else had created to show energy price and inspiration struck. Ten days later I have my monitor working, albeit not complete:

HEMS monitor

The monitor pulls together information from:

My electricity tariff for p/kWh

My immersun for power data (to/from: grid, solar, water, house)

My storage battery for power in/out and state of charge

My HEMS for electricity cost thresholds between different battery modes.

The gauge consists of two parts: (i) an upper electricity cost part and (ii) a lower power part.

The upper electricity cost part is effectively a big price gauge from 0 p/kWh to 25 p/kWh with a needle that moves each half hour as the price changes. It has various features:

The outer semi-circular ring (blue here) shows today’s relationship between battery mode and electricity price. Today is very sunny, so no electricity was bought from the grid to charge the battery, and this part is all blue for normal battery operation. If the days was duller and electricity was to be bought to charge the battery, then two further sectors would appear:

a dark green sector from zero upwards showing the grid prices at which the battery would be force charged from the grid, and

a light green sector showing when the battery is not permitted to discharge but may continue to charge from solar.

In inner semi-circular ring (green / yellow / red here) currently just colour-codes increasing electricity price, but will be used to show today’s prices at which car charging and water heating are triggered from the grid.

The current price per kWh is taken from Octopus’s price API, while the current cost per hour is derived both from this and the grid power from the immersun.

The needle grows from a simple dot indicating the price per kWh only when no power is drawn from the grid to a full needle when the electricity cost is 10 pence per hour or more.

The lower power part is effectively a power meter ranging from 5,000 Watts of export to the left to 5,000 Watts of import to the right. It updates every few seconds. It has various features:

The outer semi-circular ring (orange /maroon / green here) shows how power is being consumed:

orange – shows consumption by the house less specified loads

maroon – shows battery charging

blue (not shown) – shows water heating

green – shows export to the grid

The inner semi-circular ring (yellow here) shows the source of power. The sum of the sources should equal the sum of the consumers. The sources are:

maroon (not shown) – shows battery discharge

yellow – shows solar power

red (not shown) – shows grid power

The power value shows the net import or export from / to the grid, while SoC refers to the state of charge of the battery (0-100%). The combination of import power and electricity price gives the cost per hour in the top gauge.

The needle position shows net import (to the right) or next export (to the left). The needle should thus be to the left of the green sector, or to the right of the (unseen) red sector. Needle length show the full power being handled and is thus proportionate to the angle of the sector including all the colours in the lower gauge and extends from 0 to 5 kW.

Monitor installed on an old phone in the kitchen.

The gauge scales to fill the smallest of screen height or width and translates to be centrally positioned regardless of screen size. My intention is to display it on an old mobile phone as an energy monitor, but I can also access it on any web browser on any device within the home.